The cathodes used in the electrolytic refining of copper usually consist of starting plates of copper, which are connected to carrying bars having a length which exceeds the width of the cathode plate so that the carrying bars can be supported on the rim of the cell and an electric contact is made.
The starting plate as well as a pair of anode plates consisting of unrefined copper are immersed in a vertical orientation into the electrolyte of an electrolytic cell, or the starting plate is disposed between two insoluble anodes and together with the latter is immersed into the electrolyte of a cell for the electrowinning of metal.
Starting plates of copper must be manufactured by a special electrolytic process and cannot be reused because the copper deposited on them cannot be stripped off. In the prior art that disadvantage has been avoided by the provision of permanent cathodes consisting of starting plates made of titanium or a special steel, e.g. stainless steel.
Such a cathode made of stainless steel is particularly known from Open German application Ser. No. DE-OS 30 03 927 and is provided with a durable oxide layer, which facilitates and simplifies the removal of the copper that has been deposited on the plate whereas the oxide layer sufficiently retains the deposited copper layer as it is build up.
To permit a stripping of the copper in a simple operation, particularly by means of a machine, the copper deposit must not continuously embrace the edges of the cathode, particularly its vertical edges. It is known from Open German application Ser. No. DE-OS 30 03 927 that such difficulties can be avoided by providing at least those side edges of the starting plate which are at right angles to the carrying bar as a longitudinally grooved section bar, which is made of plastic and is held in position on the starting plate by means of plastic pins and additional adhesive joints. The plastic used in the known bar is a blend of polycarbonate and an acrylonitrile/butadiene/styrene (ABS) copolymer.
In the electrode for an electrolytic deposition of metals which is known from German Pat. No. 28 43 279 the corners of the metal plate are rounded and the insulation consists of a continuous insulating strip, which has been bent around the rounded corners. The material of the insulating strip has been forced by hot pressing into bores formed along the edges.
The cathode for an electrodeposition of metals which is known from French patent specification No. 2,388,062 is formed in its edge faces with a milled groove, in which an insulating strip made of a plastomer or elastomer has been fixed e.g., by press-forming. The outer portion of the insulating strip protrudes from the beadlike edge portions. For a deposition of, e.g., copper, the cathode may be made of stainless steel. As the beadlike portions for retaining the insulating strip are spaced different distances from the anode plate, the current density is locally increased and the deposition adjacent to the edges is undesirably increased. Besides, the assembly has an inadequate stability and involves high manufacturing costs.
The previously known arrangements comprising insulating materials, which usually consist of preshaped sections and are applied around the outer edges of the electrodes and are clamped and/or stuck thereto and in some cases are additionally fixed by mechanically means, such as rows of rivets, have the following disadvantages, inter alia:
The gap between the plate and the section member is open toward the ion stream.
The adhesive and the primer which initially fill the gap and are intended to ensure the bond between the metal plate and the plastic section fail after prolonged use owing to chemical, thermal and mechanical actions.
Owing to the differential thermal expansion of the plate material and the insulating material (plastic section), the cyclic temperature changes occurring during the periodic operation of the electrolytic cell give rise to deformation, to a formation of gaps and to shear fractures due to stresses, contraction and relaxation.
Owing to the growth of the deposited copper under the insulating section and into the gap which is open toward the ion stream, the cathode which has been formed is caught and can be stripped only with difficulty and a strong additional stress is imposed on the insulating material.
The irregular growth and the removal of defective electrodes involve a loss of operating time and a relatively high cost for repairs.